1. Field of the Invention
The present invention relates to a filter for carrying out a so-called half-pixel filtering of a picture block according to MPEG standards.
2. Description of the Related Art
Half-pixel filtering consists of processing the pixels of a picture block, or pixel matrix, in order to replace a specific pixel of the matrix with the sum of this pixel, of the adjacent right pixel and the adjacent bottom pixel. The filtered matrix obtained by processing all the pixels in this manner corresponds to a matrix that is shifted-down and shifted-right by one half pixel, hence the terminology "half-pixel filtering." In addition, the filter can be controlled to not filter at all, or to shift pixels only to the right or only to the bottom.
FIG. 1 represents a portion of an MPEG decoder in which a half-pixel filter 10 is used. The output of filter 10 and the output of an inverse discrete cosine transform circuit (DCT.sup.-1) 12 are summed by an adder 14 to provide the pixels of a reconstructed image block MBr. The DCT circuit 12 receives pixels from a picture block MBc to be decoded, and filter 10 receives the pixels of a so-called predictor block MBp fetched from a previously reconstructed picture.
The position of this predictor block in the previously reconstructed picture is found with a motion estimation vector. The horizontal and vertical components of this vector can have non-integer values. The upper left corner of the block to be processed is determined by the integer parts of the components; the half-pixel filter shifts or does not shift the block by one half pixel to the right or downward depending on whether the horizontal or vertical component of the vector is integer or non-integer.
The processed picture blocks are in fact macro-blocks including separate luminance pixels Y, and chrominance pixels U and V.
FIG. 2 illustrates an exemplary format, referred to as 4:2:0, of a macroblock MB. Macroblock MB includes a luminance matrix Y of 16.times.16 pixels constituted by four sub-matrices Y0-Y3 of 8.times.8 pixels, and a chrominance matrix of 16.times.8 pixels constituted by two sub-matrices U and V of 8.times.8 pixels. Another possible format is referred to as 4:2:2 in which the chrominance matrix includes two sub-matrices U and V of 16.times.8 pixels.